For inkjet printing on containers, such as beverage bottles or the like, e.g. EP 2 669 088 A1 and DE 10 2011 113 150 A1 disclose that containers to be printed on are guided along circular conveying paths past stationary printing stations or that printing stations circulate together with containers on carousels or the like. Due to a rotation of the containers about their own axis, a feed of the container sidewalls to be printed on is then caused in front of the respective activated print heads.
The print heads used for this purpose normally have nozzle rows extending transversely to the printing direction. Depending on the respective structural design, individual ones of these print heads may perhaps not cover the whole width of a print image as defined in a direction transversely to the printing direction. In this case, print heads will be used, whose print areas abut on one another or overlap one another in a direction transversely to the printing direction. Depending on the accuracy of alignment of print areas adjoining one another in this way, visible transitions impairing the print image will occur between the thus produced subprints, said transitions occurring e.g. at locations that have been printed on twice or at connection gaps in the print image.
In order to counteract these problems, e.g. US 2004/0252152 A1 and US 2011/0012949 A1 disclose that subprints abutting on one another in a direction transversely to the printing direction are provided in an overlapping mode, and that the transition areas are configured to interleave with one another so as to disguise double-print areas and connection gaps. The demands on the highest possible degree of precision in the alignment of neighboring print heads can thus be reduced, in particular since the relative position of the print heads and containers transversely to the printing direction can normally be observed in a reproducible manner and does not change during the printing process.
A still existing problem is, however, that, when containers are to be printed on directly, it will be necessary to print on a plurality of components of a color model over a predetermined circumferential area of the containers, and perhaps even over the full circumference thereof, making use of different print heads. In addition, due to the machine performance demanded in beverage filling plants and due to the resultant conveying speeds, it is often such that only circumferential subareas of the container surface to be printed on can be positioned in front of a specific row of nozzles and printed on continuously without any interruptions. It follows that, in many cases, subprints have to be joined together also in the direction of printing so as to produce on the containers a continuous print image in said direction of printing.
The above situation becomes more difficult due to dimensional tolerances, which, depending on the actual cross-section of the container, have the effect that the sidewall to be printed on will vary in length in the circumferential direction. Depending on the dimensional tolerances and the size of the print image extending along the circumference, the problem of an unsatisfactory quality of connection areas between subprints arises especially in the direction of printing, said unsatisfactory quality being caused by overlapping double-print areas and/or by connection gaps.
Hence, there is a need for methods and devices for inkjet printing on containers, by means of which at least one of the above-mentioned problems can be eliminated or rendered less serious.